1. Field of the Invention
The present invention relates to spacecraft Doppler tracking with a transceiver and, more specifically, is a method and apparatus for obtaining measurements on a spacecraft at intervals that are shorter than the telemetry frame duration and that may be precisely time tagged on the ground and used in the determination of precise two-way Doppler measurements.
2. Description of the Related Art
In conventional two-way spacecraft navigation, an uplink carrier is transmitted from a ground station at a given frequency. The uplink frequency is stabilized by a complex hydrogen-maser-based frequency standard. The frequency received by a coherent spacecraft transponder is then multiplied by a known factor and a downlink carrier is transmitted at the product frequency. This technique ensures that the downlink frequency maintains the high stability of the uplink frequency. The uplink and downlink frequencies are Doppler-shifted by the relative motion between the spacecraft and the ground station. However, because the frequency transmitted from the ground station and the multiplication factor applied to the received signal at the spacecraft are known, the Doppler component of the signal received from the spacecraft, and thus the relative velocity of the spacecraft, can be calculated. Such coherent Doppler tracking can be done to a high degree of accuracy assuming that the ground station frequency standard is highly stable over the round-trip propagation time.
Unfortunately, transponders on spacecraft utilizing coherent tracking are typically heavy and expensive. Furthermore, the coherency requirement is a barrier to revolutionary changes in transponder design required by the present-day emphasis on smaller, lower cost spacecraft missions. These considerations make the use of a transceiver attractive. A transceiver generates the downlink carrier frequency independent of the uplink carrier frequency. Such a system is xe2x80x9cnoncoherentxe2x80x9d because no fixed phase relationship exists between the uplink and downlink signals at the spacecraft.
When using a transceiver, the spacecraft carries a low-mass oscillator for use as a reference in generating its downlink frequency. Such spacecraft oscillators are inherently less stable however than their ground-based counterparts and are subject to significant drift errors due primarily to crystal aging and to the large temperature fluctuations found in space. The instability of the spacecraft oscillators can result in significant errors in the spacecraft velocity measurements. Therefore, a correction must be made to the Doppler frequency measured on the ground. Early ideas for reliable noncoherent spacecraft navigation were limited or required significant changes to ground station equipment to support special downlink signaling formats designed to cancel spacecraft oscillator drift.
U.S. Pat. No. 5,745,072, incorporated by reference herein, discloses a method and apparatus for two-way noncoherent measurement of spacecraft velocity that provides cancellation of oscillator drift effects and is compatible with existing ground station equipment. The ""072 patent discloses a method of two-way, noncoherent Doppler tracking wherein the uplink and downlink signals are compared within the spacecraft by periodically sampling the values held by two counters that are included in the spacecraft hardware for this purpose. This comparison is used to correct the Doppler measurement made on the ground to determine the velocity of the spacecraft. Advantages of the invention disclosed in the ""072 patent include the fact that no changes are required to ground station hardware, that an ultra-stable oscillator is not necessarily required on the spacecraft, that two-way tracking noise is eliminated, and that the spacecraft oscillator drift can be characterized during flight.
A second patent, U.S. Pat. No. 5,995,039, incorporated by reference herein, discloses a method of time-tagging two-way, noncoherent Doppler measurements when the measurements are made at the rate of once per telemetry frame. The ""039 patent discloses the addition of logic that uses a signal that is synchronous with the telemetry frame timing to trigger the sampling of the values of the two counters. The counters are used to compare the uplink frequency observed at the spacecraft with the on-board frequency reference used to generate the downlink signal.
According to the ""039 patent, the uplink carrier is downconverted in the receiver and used to provide a signal that is the clock of the first counter. A multiple of the on-board frequency reference is used to provide the clock signal for the second counter. Both counters are reset to zero at the start of a given downlink telemetry frame and are free running thereafter. Following the start of each telemetry frame, both counter values are latched, using first and second latches, on the next clocking edge of the first counter""s clock. The latched values are then put into the telemetry for use on the ground. Because it is a noncoherent system, the change in phase of the signal received on the ground will differ from what would occur with the use of a coherent system. However, the counters provide a means to compare the phase change actually measured on the ground to the phase change that would have been observed using a coherent system. The system is therefore potentially as accurate as two-way coherent Doppler tracking.
The method described in the ""039 patent is not suitable to the achievement of precise Doppler velocity measurements when the telemetry data rate becomes very low and the time between the start of successive telemetry frames becomes very long. Such infrequent measurements result in large interpolation errors in the ground processing. Interpolations are necessary because the intervals over which measurements are made on the spacecraft do not coincide with the intervals over which measurements are made on the ground. For example, in one particular spacecraft the navigation data as contained in the secondary header of a telemetry frame would be updated at the DSN (NASA""s Deep Space Network) every 13 minutes. That is too infrequent to adequately interpolate and correlate with ground-based measurements made every 0.1 to 1.0 seconds.
There is therefore a need for providing two-way, noncoherent spacecraft Doppler measurements at a rate that is greater than the telemetry frame rate, and in such a way that the measurements can be accurately time-tagged and reliably used on the ground.
The present invention is directed to a method and apparatus for providing two-way, noncoherent spacecraft Doppler measurements at a rate that is greater than the telemetry frame rate. The invention is particularly useful in deep space flight missions or in other applications where the update of navigation counters may be required to be at a rate greater than the telemetry frame rate.
According to the present invention, samples of navigation counters are triggered on a spacecraft that employs a transceiver at intervals that are shorter than the duration of a telemetry frame; the samples are then included in a telemetry frame and are time tagged after they are received on the ground; finally, the time tagged samples are used to calculate precise two-way Doppler measurements.